Electrostatic printing



Oct. 29, 1957 H. G. GRElG ELECTROSTATIC PRINTING Filed April 30, 1952 2 Sheets-Sheet l INVENTOR.

JITTORNEY Oct. 29, 1957 H. G. GREIG 2,811,465

ELECTROSTATIC PRINTING Filed April 30, 1952 2 Sheets-Sheet 2 INVENTOR.

dirty x1 TTOR NE 1 United States Patent Qfice 2,81 1,465 Patentedoct. 29, 1957 ELECTROSTATIC PRINTING Harold G. Graig, Princeton, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application April 30, 1952, Serial No. 285,260

Claims. (Cl. 117-17.5)

The present invention relates to electrostatic printing, and, more particularly, although not necessarily exclusively, to novel means for applying electroscopic developer material on a surface bearing an electrostatic image.

In accordance with the present invention a surface of a selected kind is provided with a coating of developer material which is transferred directly to a surface bearing an electrostatic image. The coating may be superficial or quantities of the developer material may be contained in depressions in the surface. Transfer of the developer material to the electrostatic image from the surface bearing the developer material may be, and is preferably, accomplished by direct contact between the two surfaces. in preferred embodiments of the invention, the surface carrying the developer material is continuous, as, for example, the periphery of a cylinder, and thereby continuous development and concomitant printing of images is attained.

Development and printing of electrostatic images on a surface in accordance with the present invention is a more direct and more easily controlled procedure than previously known cascade processes or development by droplet or powder cloud. Control and replenishment of developer material are serious problems in the case of cascade development, and control of the cloud and polarity of charges on the cloud particles are problems in the powder cloud process.

The principal object of the present invention is to provide novel ethods of and apparatus for applying developer material to a surface bearing an electrostatic image.

Another object of the present invention is to provide novel means for applying developer material directly from a surface coated with or retaining developer material to a surface bearing an electrostatic image.

A further object of the present invention is to provide novel methods of and apparatus for applying developer material in a continuous manner to a surface or plurality of successive surfaces bearing an electrostatic image or images.

A still further object of the present invention is to provide arrangements bearing an analogy to planographic or intaglio printing processes for applying electroscopic developer material to a charged surface.

Still another object of the present invention is to provide novel apparatus for automatically obtaining continuous printing of a series of electrostatic images.

Other obiects and advantages of the present invention will, of course, become apparent and immediately suggest themselves to those skilled in the art to which the invention is directed from a reading of the following specification in connection with the accompanying drawing in which:

Fig. 1 is a perspective view of, apparatus embodying the present invention for applying a developer material such as an electroscopicpowder to an electrostatic image bearing surface;

Fig. 2 is a view in perspective of a modified embodiment for applying developer material to an electrostatic image bearing surface;

Fig. 3 is a diagrammatic showing in perspective of continuous printing apparatus embodying the invention; and

Fig. 4 is a diagrammatic showing in side elevation of another embodiment of the invention for effecting continuous printing.

The present invention is particularly adapted for use in developing electrostatic images which are provided on the improved carrier base which is set forth in the copending application for Letters Patent of the United States of Harold G. Greig, Serial No. 248,937, filed September 29, 1951, now abandoned. In accordance with that application, an electrostatic reproduction process and a carrier base is disclosed in which the use of a photosensitive plate is eliminated. A surface of the car rier base which is to receive the copy is given a coating of a composition comprising, for example, zinc oxide powder of a relatively pure grade suspended in a vehicle composed of an electrically insulating, film-forming substance. An electrostatic charge is applied to the coated surface and a light image of the nature to be reproduced is directed onto the charged surface causing a charge image to be formed which corresponds to the light image. This charge image may be stored for a time in the dark, if desired, or it may be immediately developed to render it visible. Development in accordance with the invention described in the above-identified application may be carried out by applying an electroscopic developing substance such as a pigmented powder which adheres to the charged portions and does not adhere to the discharged portions. Alternatively, the developer powder may be one which adheres to the discharged portions of the charge image. A preferred example of the image carrying base disclosed in the application above identified will be given for the sake of completeness of disclosure, although it is not a part of the present invention:

60 gms. of a 10 percent solution of polyvinyl acetate resin in methanol 40 gms. zinc oxide, C. P. (made by a dry process) 20 ml. of acetone as thinner 'These ingredients were mixed for 1 hour in a porcelain ball mill to obtain a smooth, uniform m-ix. Ordinary white bond paper was coated and impregnated by immersion in the mix, removing the impregnated paper, draining off excess composition and air drying. The mix may also be applied as a surface coating by thinning with acetone and spraying with a spray gun, or by any other of the usual paper coating techniques, such as flow coating, roller coating, and the like.

in obtaining a print in accordance with the aboveidentified application, a negative charge is applied in the dark over one surface of the resulting, coated paper. Following this, a light image is projected onto the paper so that, wherever the light strikes the charged coating layer the electrostatic charge is lowered or removed. The charge image is then dusted with an electroscopic developer powder. The developer image is fixed by heating.

The preferred developer powder for use with the coated paper just described is disclosed in the copending application for Letters Patent of the United States of Harold G. Greig, Serial No. 187,827, filed September 30, 1950. Gne form of this developer powder may be obtained by taking a fine powder having good dielectric properties, such as sublimed sulphur, and forming it into a thin slurry or paste by mixing 40 parts by weight of the sulphur with 50 parts by weight of methanol. 1.5 parts by weight of spirit-soluble nigrosine dye SSE, color index No. 864,

is added to the paste and the mixture stirred until the dye is completely dissolved in the alcohol. To this is then added 1 part by weight of Iosol black or any other dye which will react with the nigrosine to precipitate a black dye. The product is isolated by filtering, washed with a small amount of methanol and dried. This powder can either be used as is, or after grinding in a ball mill, to enhance its electroscopic properties.

The type of powder above described is a positively charged powder and will readily adhere to negatively charged areas of a charge image. Negatively charged powders are dyed or pigmented powdered synthetic or natural resins. Examples of synthetic resins are coumaronc-indene resins and Vinsol. Examples of natural resins are gum copal and rosin.

There are at least two general methods by which latent images, in the form of an electrostatic charge on a surface, can be developed according to the present invention with electroscopic powder carried on the surface of a powder applying member. The powder applying member is most conveniently a roller or a plurality of associated rollers.

In the first of the general methods, the powder is carried on the smooth surface of the powder applying roller. The surface material of the roller may be either hard or soft and may be either a conductor or an insulator. The powder may be held on the surface of the roller by electrostatic charge or possibly in some instance by tackiness of the roller surface material. With a roller having a conductive surface and the powder being an insulator, the charge can be applied to or intensified on the powder by corona charging on the roller, or the powder may already carry a charge when it is applied to the roller surface. When the surface is an insulator, the powder may gain its charge by triboelectric effect on contact with the surface if the materials of the surface and the powder are selected so that the charge is obtained in this manner.

In accordance with the present invention, the development procedure is to bring the surface With a latent image in the form of an electrostatic charge close enough to the roller surface so that the powder is lifted from the roller and held on the surface by the electrostatic attraction. Normally, rolling the two surfaces in direct contact is sufiicient, and, in some instances, considerable pressure can be applied to insure uniform contact. Two conditions for development are that the powder on the roller should carry a suitable electric charge of the polarity required by the electrostatic charge on the surface; and, also, that the charged powder should be held on the roller surface with force sufilcient to permit selective removal of the powder by the charges on the surface. Considering the first condition further, if the image areas are to be developed, then the powder and image should carry charges of opposite polarity and a direct print results. For example when using the paper described above, which is fully disclosed in the application Serial No. 248,937, now abandoned, with a negative charge originally applied, then the image areas are negatively charged and the powder described above and referred to as a positively charged powder will readily adhere to negatively charged areas of the charge image. Conversely, if the background is to be developed, then the image and the powder should carry like charges and a reverse print is obtained. For example, if a blanket negative charge is applied to the coated paper and the image is in the form of illuminated type characters, then the type images will be neutral or positively charged. A negative powder, such as pigmented rosin, can be applied.

The first general method, just discussed, of this invention is somewhat analogous to planographic printing; The powder coated roller corresponds to the ink coated roller of the printing press with the powder being held on the surface by electrostatic forces instead of by the wetting characteristics and tackiness in the case of the ink. The plane surface to be printed in the printing process gains its differential from differences in the wetting characteristics between water and grease (hydrophilic and hydrophobic surfaces). In the image developing process of this invention the dilferential is between the differences of electrostatic charges.

More specifically and by way of example image development on the electrophotographic coated paper described above and also disclosed in full detail in application Serial No. 248,937, above referred to, has been obtained by rolling the charged and exposed print between two rollers or in contact with the surface of a single roller. A suitable arrangement of rollers where a backing roller is used is indicated by Fig. 1 of the drawing. A stand having a base member 10 provided with uprights 14 and 15 serves as a support for the axle 17 of the roller 18. A second roller 21 is carried by an axle 22 which is journalled in extensions 24 and 26 of the uprights 14 and 15. The extensions 24 and 26 are insulated from the uprights 14 and 15 each by a layer of insulating material 27 and 28. A hand crank 31 is provided so that the roller 18 may be turned. The axle 22 of the roller 21 is received in slots in the extensions 24 and 26. Each slot may have a more or less tightly fitting filler 33 of insulating material. Parts 14, 15, 24, and 26 may be unitary and grounded.

The developer roller 21 serves to apply the electroscopic powder to the surface of a light exposed sheet which is passed between the rollers 18 and 21 by turning the crank 31. The roller 18 is conductive and may be made of metal to serve as a backing roller. The roller 18 may be omitted and the paper or the like bearing the charged image or images may be passed over the roller 21 to obtain development. Also a roller, such as the roller 21, of any of the forms discussed herein as part of the present invention may be coated with electroscopic powder and may be rolled over a flat surface or a flattened sheet bearing a latent electrostatic image or images.

The roller 21 may be a relatively soft roller such as a printers hand proof inking roller. By way of example this roller may be cast from a mix such as casein, glycerin and sugar. It may also be cast from glue, molasses and glycerin. The resulting roller is soft, hygroscopic and electrolytically conductive. The soft cast roller mentioned by way of example is suitable unless considerable humidity is present in the atmosphere. A preferred roller is one of rubber, for example, natural or synthetic, which is conductive but not electrolytically conductive.

The roller 21 of Fig. 1 may be made of an insulated drum coated with an insulating lacquer. An acrylic acid resin known as Krylo'n is suitable for this purpose. Krylon is available from Krylon, Inc., 2601 North Broad Street, Philadelphia 32, Pa.

Prints may be developed in accordance with the invention by employing the apparatus of Fig. 2 which includes a roller 36 which may be of the soft type just discussed. The roller 36 may also be of the pitted conductive type to be discussed. The shaft 38 of this roller is supported in insulated end frame members 41 and 42. Fine wires 44 are connected as indicated at reference character 46 to a suitable source of high voltage (not shown). A conductive plate 48 of metal, for example, is grounded as shown. Powder is applied to the roller 36 and a charge is imparted to this powder by the high voltage applied to the corona wires 44. Transfer to the sheet bearing the electrostatic image may then be effected by difference in charge between the powder and the charges on the sheet.

The second general method according to the invention by which electrostatic images on a surface can be developed with electroscopic powder carried by a roller in- Volves use of a grained or grooved surface on the roller. Referring again to Fig. 1 the roller 21 may be of metal or it may have an insulating surface with a multiplicity of pockets provided in it. The powder is carried in the pockets While high points are cleaned of powder. The grained or grooved or pocketed developer roller is rolled in contact with the surface bearing the electrostatic image,

and the charge powder is attracted by the charged surface areas but is retained in the pockets as the roller passes over uncharged or reduced charge areas.

When the developer roller has a surface which is an insulator a material should be chosen which imparts the desired charge to the developer powder by triboelectric effect on contact. When the metal roller is selected the powder may be charged before it is fed on the roller or may gain triboelectric charge by contact with the metal.

This development process is analogous to intaglio printing. A blanket surface of small depressions carrying powder is presented to the surface to be printed and the differential for removal of the powder is supplied by the differences in charge on this surface. In the intaglio printing process on the other hand, the ink is presented in depressions only in the image areas and blanket removal of the ink is obtained.

The roller 21 may be made of brass and knurled with a diamond knurling tool. It is then faced on a lathe to obtain an even surface of high points. In use, this roller was used as the roller 21 in Fig. 1. In a test of this roller, powder was brushed into the pockets with a camels hair brush and the high points were cleaned of powder by doctor blade action of a rubber surface. Prints of high definition were obtained by rolling the charged, exposed electrophotographic paper between the developer roller 21 and a metal backing roller 18. Fine grain and more uniform surface with controlled depths of pockets can be obtained by etching the roller surface through a dot pattern resist or by engraving the roller surface.

Fig. 3 shows diagrammatically an arrangement for obtaining continuous printing on the electrographic paper described above with roller development of the image. Electrographic paper from the supply roll 51 passes over the grounded roller 52 under the corona charge unit 53 where it receives a blanket of electrostatic charge in the dark. The charged paper is exposed to a light image at 56 from a luminous image projector 58 of any known kind. The paper bearing the latent image in the form of an electrostatic charge then passes between the series of developer rollers 61 and 62 associated with the backing rollers 64 and 65. The colored powder carrying a charge is selectively attracted to the surface of the paper in the charged image areas or in the background depending on whether the powder carries a charge of opposite polarity to that of the image or the same polarity as described above. The developer rollers 61 and 62 are continuously fed with developer powder from powder feeding hoppers 69 and 70. When the rollers are of the grained surface type disclosed above, the high points are cleaned of powder by the doctor knives 72 and 73. The developed paper carrying the loose powder image then passes through the fuser 76 where the powder is melted and fused to the paper surface to form a permanent image. The printed paper then passes between the drive rollers 78 and 79 and can be cut to provide finished prints.

Fig. 4 of the drawing is a modification of the arrangement of Fig. 3 where the backing rollers 64 and 65 are replaced by a metal drum roller 86. This drum roller is grounded and paper 88 is fed from a supply roll 89 past the corona charger device 91. A suitable image projector 93 of any known type projects a luminous image on the face of the web 88 following which the web passes beneath the developer rollers 96 and 97. Powder is continuously supplied to these developer rollers from powder containing hoppers 101 and 102. Doctor knives 104 and 106 serve to clean the powder from the developer rollers except where it is retained in the grooves. The paper web 88 is locked on the drum by the electrostatic charge imparted by the corona charger 91 throughout the processing and is peeled off to pass through a fuser 108 which fixes the images on the web 88. Any known means may be employed to drive the drum 86 by power applied rotatively to its grounded supporting shaft 110. The web 88 after it is peeled from the roller is drawn through the fuser 108 over an idler roller 112 by any suitable tractive means (not shown).

What is claimed is:

l. A device for applying electroscopic developer material to a record receiving medium bearing a latent electrostatic charge image to be developed, said device comprising a cylindrical member presenting a peripheral surface capable of retaining a quantity of electroscopic ma terial thereon, means for uniformly distributing a quantity of electroscopic developer material on said surface, and means for bringing said surface into contact with the surface of the record receiving medium.

2. A device for applying electroscopic developer material to a record receiving medium bearing a latent electrostatic charge image to be developed, said device comprising an electrically conductive member presenting a substantially cylindrical surface capable of retaining a quantity of electroscopic material thereon, means for uniformly distributing a quantity of electroscopic developer material on said surface, and means for bringing said surface into contact with the surface of the record receiving medium.

3. The device of claim 2 wherein said electrically conductive member bears a coating of an insulating lacquer, said lacquer coating presenting the surface capable of retaining the electroscopic material thereon.

4. A device for applying electroscopic developer material to a record receiving medium bearing a latent electrostatic charge image to be developed, said device comprising a yieldable member electrically conductive presenting a substantially cylindrical surface capable of retaining a quantity of electroscopic material thereon, means for uniformly distributing a quantity of electroscopic developer material on said surface, and means for bringing said surface into contact with the surface of the record receiving medium.

5. A device for applying electroscopic developer material to a record receiving medium bearing a latent electrostatic charge image to be developed, said device comprising an electrically conductive member, said member presenting a substantially cylindrical surface having depressions therein capable of retaining a quantity of electroscopic material, means for uniformly distributing a quantity of electroscopic developer material on said surface, and means for bringing said surface into contact with the surface of the record receiving medium.

6. A device for applying electroscopic developer material to a record receiving medium bearing a latent electrostatic charge image to be developed, said device comprising a cylindrical roller member, means for uniformly coating said roller member with said electroscopic developer material, said developer material being held onto said roller by electrostatic attraction, means for guiding said record receiving member into intimate rolling contact with said roller whereby to transfer, through an electrostatic attraction, between said charge image bearing record receiving member and said developer which is greater than said first mentioned attraction, portions of said developer material from said roller onto selected areas of said record receiving medium corresponding to said charge image.

7. A device for applying electroscopic developer powder to a record receiving medium bearing a latent electrostatic charge image to be developed, said device comprising a cylindrical roller member having a surface capable of retaining a uniformly distributed quantity of electroscopic developer powder thereon, means for uniformly distributing a quantity of said developer powder on said roller member, said powder being held onto said roller by electrostatic attraction, and means for passing the record receiving medium into intimate rolling contact with said roller whereby to transfer, through an electrostatic attraction, between said powder and said charge image bearing member which is greater than said first mentioned attraction, portions of said developer pow- 7 der from said roller onto selected areas of said record receiving member corresponding to said charge image.

8. A device for applying electroscopic developer powder to a record receiving medium bearing a latent electro'static charge image to be developed, said device comprising a cylindrical roller member having a surface provided with a multiplicity of small pits substantially uniformly distributed about said surface, means for applying a quantity of electroscopic developer powder to said roller, said powder being received and held by an electrostatic attractive force in said pits, and means for passing the record receiving medium into intimate rolling contact with said roller member whereby to transfer, through electrostatic attraction greater than said first mentioned attractive force, portions of said developer powder from said roller onto selected areas of said record receiving member corresponding to said charge image.

9. An electrostatic printing system comprising means for advancing and supporting a member presenting a photoconductive surface, means for imparting a blanket electrostatic charge to said surface, means for projecting a light-image onto said charged surface to discharge selected areas of said surface in accordance with said light image leaving a latent electrostatic charge image on said surface, means for developing said latent image, said developing means including a cylindrical roller member having a surface provided with a multiplicity of small pits substantially uniformly distributed about said roller surface, means for applying a quantity of electroscopic developer powder to said roller member, said powder being received and held by an electrostatic attractive force in said pits, and means for passing said record receiving member into intimate rolling contact with said roller member whereby to transfer, through an electrostatic attraction, greater than said first mentioned attractive force, portions of said powder from said roller memberonto selected areas of said record receiving member corresponding to said charge image. 1

10. The method of developing latent electrostatic charge images on a record receiving member comprising the steps of applying a substantially uniform distribution of electroscopic developer powder to the surface of a substantially cylindrical surface capable of retaining, through an attractive force, a quantity of said powder thereon, charging said powder by exposure to a corona discharge, rolling said surface in contact with the surface of a record receiving member bearing a latent electrostatic charge image, and transferring portions of said developer powder to areas of said record receiving member corresponding to said charge image during said contact through an electrostatic attractive force established by said charge image which is greater than said first mentioned attractive force.

References Cited in the file of this patent UNITED STATES PATENTS 1,784,912 Scott Dec. 16, 1930 2,302,179 Bronfman a Nov. 17, 1942 2,332,514 Holtzclaw Oct. 26, 1943 2,345,941 Lehman Apr. 4, 1944 2,357,809 Carlson Sept. 12, 1944 2,483,462 Huebner Oct. 4, 1949 2,618,551 Walkup Nov. 18, 1952 2,618,552 Wise Nov. 18, 1952 

10. THE METHOD OF DEVELOPING LATENT ELECTROSTATIC CHARGE IMAGES ON A RECORD RECEIVING MEMBER COMPRISING THE STEPS OF APPLYING A SUBSTANTIALLY UNIFORM DISTRIBUTION OF ELECTROSCOPIC DEVELOPER POWDER TO THE SURFACE OF A SUBSTANTIALLY CYLINDRICAL SURFACE CAPABLE OF RETAINING, THROUGH AN ATTRACTIVE FORCE, A QUANTITY OF SAID POWDER THEREON, CHARGING SAID POWDER BY EXPOSURE TO A CORONA DISCHARGE, ROLLING SAID SURFACE IN CONTACT WITH THE SURFACE OF A RECORD RECEIVING MEMBER BEARING A LATENT ELECTROSTATIC CHARGE IMAGE, AND TRANSFERRING PORTIONS OF SAID DEVELOPER POWDER TO AREAS OF SAID RECORD RECEIVING MEMBER CORRESPONDING TO SAID CHARGE IMAGE DURING SAID CONTACT THROUGH AN ELECTROSTATIC ATTRACTIVE FORCE ESTABLISHED BY SAID CHARGE IMAGE WHICH IS GREATER THAN SAID FIRST MENTIONED ATTRACTIVE FORCE. 